A flange bolt is a hex bolt with an integrated circular flange beneath the head that acts as a built-in washer, distributing clamping load over a wider surface area. It comes in two types (serrated for vibration resistance and non-serrated for surface protection) and is manufactured to DIN 6921 and ISO 1665 dimensional standards. The integrated flange eliminates the need for a separate washer, reducing parts count and assembly time. CNXD manufactures flange bolts in carbon steel grades 8.8 and 10.9 and stainless steel grades A2 and A4, under IATF 16949 quality management.
What is a flange bolt? Definition and Design
The unique aspect of the flange bolt is the flange itself, which is a broad plate between the hex head and bearing face.
Once torqued, the flange distributes the stress over a larger area than the standard hex bolt head can distribute.
This prevents the fastener from splitting or tearing through when stressed because of the larger contact area.
Flange bolts are made in accordance with DIN 6921 and ISO 1665. In both standards, the dimensions, pitch of thread, and mechanical specifications of the fasteners are detailed for various sizes.
In a flange bolt, the thread starts directly under the flange and extends along the entire shank length.
What are the Types of flange bolts?
There are two types, but the distinguishing feature is the underside of the flange.
Serrated flange bolts
Serrated flange bolts have fine radial teeth cut onto the underside of the flange.
Due to their design, when tightening them, the teeth dig into the mating surface, thus preventing any loosening due to vibrations. The self-locking feature is widely used in automotive engine assemblies and suspensions.
Applications where there will be continuous vibration would require the use of the serrated type of flange bolt.
A disadvantage of the serrated design is the ability of the teeth to damage the substrate surface if it is made of softer material such as aluminum.
If there is a requirement for no surface marking, then it is better to use non-serrated flange bolts.
Non-Serrated flange bolts
Non-serrated flange bolts have a flat underside and are used when it is required to protect the mating surface or when repeated installation and removal are expected.
Steel structural connections, HVAC equipment, and other machinery assemblies are typical applications for smooth flange bolts.
Flange Bolt Dimensions Chart
| Thread Size | Pitch (mm) | Head Width Across Flats (mm) | Flange Diameter (mm) | Standard Length Range (mm) |
|---|---|---|---|---|
| M5 | 0.8 | 8 | 14.2 | 10 to 50 |
| M6 | 1.0 | 10 | 17.0 | 10 to 60 |
| M8 | 1.25 | 13 | 22.0 | 12 to 80 |
| M10 | 1.5 | 17 | 27.0 | 16 to 100 |
| M12 | 1.75 | 19 | 32.0 | 20 to 120 |
| M14 | 2.0 | 22 | 37.0 | 25 to 140 |
| M16 | 2.0 | 24 | 41.0 | 25 to 150 |
| M20 | 2.5 | 30 | 50.0 | 30 to 200 |
Standard flange bolts follow DIN 6921, which covers metric thread sizes from M5 through M20. The table below shows key dimensions for the most commonly used sizes.
Flange bolt dimensions vary slightly between manufacturers, so always verify against the relevant DIN 6921 specification sheet when tolerances are tight.
Flange Bolt vs Hex Bolt: When to Use Which
The flange bolt vs hex bolt decision usually comes down to two factors: assembly speed and load distribution.
When a Flange Bolt is a Superior Choice
Flange bolts are superior for applications requiring fast assemblies.
Since they eliminate the need for a washer, they have fewer parts and take less time to assemble, which is important when working on vehicle assembly lines.
The flange ensures proper alignment of the washer, thereby offering consistent clamping force on the bolt.
For vibration-sensitive applications, serrated flange bolts are preferred for their built-in locking feature, eliminating the use of locking washers and compounds.
When a Hex Bolt is the Preferred Option
There are cases where hex bolts have been found superior to flange bolts. This happens in situations where the washer material or size, and hardness specifications cannot be achieved by a standard flange.
Hex bolts work well in the construction of structural steel structures where washers are under ASTM F436 hardened standards and loads exceeding those achievable by a standard flange are present.
It is easier to obtain hex bolts in larger diameters and higher strengths than standard DIN 6921 flange bolts.
Flange Bolt Materials and Surface Treatments
There are many grades of flange bolt materials depending on the load requirement and environmental factors.
Grade 8.8 and 10.9 carbon steels comprise most automotive and general industrial uses.
Grade 8.8 is used for moderate load structures and mechanical assemblies. Grade 10.9 is applicable for heavy-load, vibration-prone systems requiring high tensile strength.
Stainless steel grades A2 and A4 are applied in corrosive situations.
Grade A2 (304 stainless steel) is for general outdoor and mild chemical-resistant situations, while Grade A4 (316 stainless steel) contains molybdenum that provides resistance against chlorides and marine exposure.
Common surface finishes are zinc plating, hot-dip galvanizing, and DACROMET. Zinc plating is the usual material in general applications, while hot-dip galvanization is applicable in outdoor structural uses.
Dacromet is known to be highly corrosion-resistant without causing hydrogen embrittlement in automotive fasteners.
Where are flange bolts used?
Automotive Manufacturing
Flange bolts are one of the most common fastener types in vehicle assembly.
Engine mounts, exhaust systems, brake calipers, and suspension brackets all use serrated flange bolts for their vibration resistance.
Most automotive OEM specifications call for Grade 10.9 in high-stress locations.
Structural and Mechanical Applications
In structural steel and mechanical equipment, non-serrated flange bolts simplify assembly by removing the separate washer step.
Frame connections, equipment bases, HVAC units, and conveyor systems are common applications. Grade 8.8 covers most of these loads adequately.
Piping and Flange Connections
In piping systems, the term “flange bolt” typically refers to studs or hex bolts that connect pipe flanges together. That is a different product from the hex flange bolt described here.
Confirming the actual fastener design before ordering avoids a common sourcing mistake.
CNXD manufactures flange bolts in carbon steel grades 8.8 and 10.9, along with stainless steel grades A2 and A4.
Zinc plating and DACROMET surface treatment options are available. All products are manufactured under IATF 16949 quality management with full material traceability per order.
- This DIN 34801 hexagon head flange bolt is manufactured from high-strength steel and stainless steel, offering exceptional tensile strength and structural stability.
FAQs
What is a flange bolt used for?
A flange bolt is used in places where a washer is required, but using an additional part will increase the assembly time or inconsistency.
This is a very commonly used part in automotive assemblies, mechanical equipment, and structures where vibration or load resistance is necessary.
How are serrated and non-serrated flange bolts different?
In a serrated flange bolt, there are teeth in the bottom part of the flange that help resist the loosening effect due to vibrations. In the case of a non-serrated version, there are no teeth but a smooth flange face.
What standard governs flange bolt dimensions?
The DIN 6921 standard deals with the dimensions of the metric hex flange bolts, for thread sizes ranging from M5 to M20. The international standard equivalent to this is ISO 1665. Always refer to the standard for any important application of flange bolts.
